誰・どなた

誰 (だれ) means "who". It takes particles like any noun: 誰が (who, as subject), 誰と (with whom), 誰に (to whom), and 誰の for "whose": これは誰のですか, "whose is this?"

As a question word it can't take は; the subject version is 誰が: 誰が来ましたか.

どなた is the polite version, used about or to someone you should be respectful to: 失礼ですが、どなたですか, "excuse me, may I ask who you are?" Asking a stranger 誰ですか to their face is blunt.

Add か for "someone" (誰か) and も with a negative for "no one" (誰もいない), covered a few points on.